The new union's in complete agreement (6)
I believe the answer is:
unison
'complete agreement' is the definition.
(unison can mean total agreement or harmony)
'the new union's' is the wordplay.
'the new' is an anagram indicator.
'unions' with letters rearranged gives 'UNISON'.
'in' is the link.
(Other definitions for unison that I've seen before include "Simultaneous action" , "Public service trade union" , "All singing the same notes" , "with regard to the pitch" , "Agreement or concord, as with choir" .)